Heat-conducting adhesive joint with an adhesive-filled, porous heat conductor

ABSTRACT

A heat-conducting adhesive compound is provided with a sintered layer ( 3 ) that consists of heat-conducting power, is arranged between two workpieces and contacts each workpiece in a two-dimensional manner. The compound is also provided with an adhesive ( 4′ ) which fills openings ( 33 ) on the surface ( 31 ) of the layer and adheres to the two workpieces. In a preferred embodiment, the sintered layer consists of silver powder. The sintered layer is produced between the workpieces and the layer is subsequently filled with a liquid adhesive ( 4 ) which is then hardened.

FIELD OF THE INVENTION

The invention relates to a heat-conducting adhesive joint between twoworkpieces and a method for producing a heat-conducting adhesive jointbetween two work-pieces.

BACKGROUND OF THE INVENTION

Electronic components, in particular power semiconductor components suchas IGBTs, MOS-FETs, diodes, thyristors and so on produce high powerlosses during operation, which have to be dissipated efficiently inorder not to exceed the maximum operating temperature.

For semiconductor chips up to at most 2×2 cm in size, soft solderingwith tin, lead and their alloys on supporting elements made of ceramicor metal has become completely widespread in the technology. Othermethods, such as those with gold solders, glass pastes and so on havefound only a very narrow field of use, for cost reasons.

Development trends are leading, on one hand, to ever higher operatingtemperatures until close to the melting points of the solders, with thereliability increased at the same time, but on the other hand leadshould be superseded for environmental protection reasons, in particularalso by law.

Adhesive bonding, which is otherwise very common in chip assemblytechnology, suffers from poor thermal conductivity and also poorelectrical conductivity of the adhesives.

This poor thermal conductivity of the adhesives can be improved by meansof particles which conduct heat well and which are suspended in thepoorly conducting adhesive. For example, DE-A-195 29 627 (95P1762 DE)discloses improving the thermal conductivity of the adhesive by adding aheat-conducting powder, for example nickel powder.

In practical terms, this document describes a heat-conducting,electrically insulating adhesive joint between two workpieces, saidjoint having a layer of ceramic material and a layer of adhesive.

The layer of ceramic material has two flat surfaces which face away fromeach other and, on each flat surface, there are openings defined byvoids in the layer, and the layer is arranged between the two workpiecesin such a way that one of the two flat surfaces of one of the twoworkpieces, which is constructed in the form of a heat sink, has flatcontact made with it. Furthermore, at least the openings on the otherflat surface, which faces away from the one flat surface, is filled withelectrically insulating material.

The layer of adhesive is arranged between the layer of ceramic materialand the other workpiece, which forms an electronic power component, andhas two flat surfaces that face away from each other. One of thesesurfaces makes flat contact with the other workpiece and adheres to thelatter. The other surface makes flat contact with the other flat surfaceof the layer of ceramic material and adheres to said layer.

In order to improve the thermal conductivity of the layer of adhesive, aheat-conducting powder, for example nickel powder, is added to saidlayer.

This known adhesive joint is produced as follows:

The ceramic layer is produced on one workpiece by means of thermalspraying, the openings defined by voids in the layer being producedautomatically on the flat surfaces of this layer.

At least the openings on the other flat surface facing away from the oneflat surface and the one workpiece are filled with electricallyinsulating material.

The layer of the ceramic material which comprises the electricallyinsulating material is joined to the other workpiece by means of theadhesive layer to which the heat-conducting powder is added.

DE 34 14 065 A1 (84 P 1304) and EP 0 242 626 A2 (86 P 1242) in each casereveal a different type of adhesive-free joint between a workpiece inthe form of an electronic component and a workpiece in the form of asubstrate, which has a layer of heat-conducting material in the form ofa sintered metal powder and is therefore both thermally conductive andelectrically conductive.

The layer of the sintered metal powder has two flat surfaces which faceaway from each other and each of which has openings defined by voids inthis layer.

The layer is arranged between the two workpieces in such a way that oneof the two flat surfaces is sintered onto one of the two workpieces andthe other flat surface is sintered onto the other workpiece.

The sintered metal powder of the layer is coherent from one of the flatsurfaces in the direction of the other flat surface.

The production of the adhesive-free joint according to DE 34 14 065 A1is carried out by the following steps:

A paste is applied to one workpiece and/or the other workpiece, saidpaste being composed of a mixture of a metal powder which can besintered at a specific sintering temperature and a liquid.

The two workpieces are brought together in such a way that the paste islocated between the two workpieces and makes contact with bothworkpieces.

The paste is dried and the dried powder is sintered by heating to thesintering temperature. This sintering is carried out in a non-oxidizingatmosphere, for example N₂ or H₂, and the sintering temperature in thiscase is about 400° C. During the sintering operation, a mechanicalpressure, for example 80 to 90 N/cm², can be exerted.

The production of the adhesive-free joint according to EP 0 242 626 A2is carried out by the following steps:

A paste is applied to a workpiece, said paste being composed of amixture of a metal powder that can be sintered at a specific sinteringtemperature and a liquid.

The paste is dried.

The other workpiece is placed on the dried powder.

The entire arrangement is then heated to sintering temperature with thesimultaneous exertion of a mechanical pressure of at least 900 N/cm².The sintering temperature is about 230° C. to 250° C.

In the thesis by Sven Klaka: “Eine Niedertemperatur-Verbindungstecknikzum Aufbau von Leistungshalbleiter-modulen” [A low-temperature joiningtechnique for the assembly of power semiconductor modules], CuvillierVerlag, Göttingen 1997, in this connection the sintering operation inthe case of silver powder is examined at low sintering temperaturesbetween 100° C. and 250° C. and it is established that this powder canform sintered bridges between 200° C. and 250° C.

SUMMARY OF THE INVENTION

The invention is based on the object of providing a heat-conductingadhesive joint between two workpieces which exhibits a higher thermalconductivity than a joint with a layer of adhesive to which aheat-conducting powder is added.

This object is achieved by the features of claim 1.

According to said claim, the heat-conducting adhesive joint according tothe invention comprises:

a layer of heat-conducting material,

which has two flat surfaces facing away from each other,

which, on each flat surface, has openings defined by voids in the layer,

which is arranged between the two workpieces in such a way that one ofthe two flat surfaces (31) makes flat contact with one of the twoworkpieces, and the other flat surface makes flat contact with the otherworkpiece, and

whose heat-conducting material is coherent from one of the flat surfacesin the direction of the other flat surface,

and

an adhesive

which fills the openings in the layer and

which adheres to both workpieces.

The term “coherent” is to be understood in such a way that, in the layerof heat-conducting material, this material coheres from one of the flatsurfaces in the direction of the other flat surface of the layer outsidethe voids of this layer, or forms a unit, at least in such a way as inthe case of a layer of sintered powder of heat-conducting material. Suchcoherence, beneficial for the thermal conductivity, is not present in athermally and electrically poorly conducting layer of adhesive to whichheat-conducting powder is added, since heat-conducting paths are formedin this layer only at comparatively few points of contact between theparticles of the added powder.

The higher the thermal conductivity of the heat-conducting material ofthe layer, the more beneficial this is for the heat-conducting adhesivejoint according to the invention.

One advantage of the adhesive joint according to the invention is thatit can be implemented as desired as an electrically conductive orelectrically insulating joint, depending on whether the heat-conductingmaterial selected for the layer is electrically conductive, for examplemetal, or electrically insulating, for example heat-conducting ceramicmaterial.

The strength of the adhesive joint according to the invention isadvantageously composed of the inherent strength of the layer ofheat-conducting material and the strength of the adhesive, can thereforebecome significantly greater than in the case of an adhesive joint ofpure adhesive or adhesive to which powder of heat-conducting material isadded. At high temperatures, the strength of the layer ofheat-conducting material generally dominates. The adhesiveadvantageously protects the layer of heat-conducting material, inparticular at high temperatures, against reaction of the layer withoxygen or another oxidizing gas.

In principle, it is sufficient if the layer of heat-conducting materialhas only voids which define openings on the flat surfaces of the layer.For example, the layer can be a film of heat-conducting material withholes, each of which defines one opening on both flat surfaces of thelayer.

The layer of the heat-conducting material is preferably interspersed inthe manner of a sponge with voids, so that there are also voids in theinterior of the layer which do not immediately adjoin the flat surfacesof the layer and do not define any openings on these surfaces.

In this case, it is advantageous if at least those voids in the layerwhich define openings on one flat surface of the layer are joined to oneanother. Through these voids, curable liquid adhesive can be introducedin a simple way from the outside into the openings on one flat surfaceof the layer, even if this surface is already in contact with aworkpiece.

It is beneficial if as far as possible all the available voids areconnected to one another, and, in relation to the curable liquidadhesive, are so small that, for this adhesive, they act likecapillaries which exert a suction action on said adhesive. In this case,the curable liquid adhesive can advantageously substantially beintroduced automatically from the outside through the layer and into theopenings on the flat surfaces of this layer by means of capillarysuction action, irrespective of whether the surfaces are already incontact with a workpiece or not. As an alternative or in addition to thecapillary suction action, the curable liquid adhesive can be introducedinto the layer with support from pressure.

In a preferred embodiment of the joint according to the invention, theheat-conducting material of the layer is selected from the group ofmetals, in particular from the group of noble and semi-noble metals. Itis particularly advantageous here for the heat-conducting material tocomprise silver.

The layer is preferably and advantageously composed of sintered metalpowder. Such a layer, which is electrically conductive, has thefollowing advantages, for example: it can be produced easily, undercertain circumstances it can be sintered onto a workpiece or onto bothworkpieces to be joined and, on its own, can already form aheat-conducting joint to one or both workpieces, which assists the jointproduced by the adhesive and is inherently formed in such a way that ithas openings defined by voids on its flat surfaces and is interspersedin the manner of a sponge with voids, the voids being connected to oneanother and, in relation to a curable liquid adhesive, being capable ofbeing so small that they exert a capillary suction action on thisadhesive, and so on.

The adhesive joint according to the invention is particularly wellsuited for fixing an electronic component, in particular a powercomponent, to a supporting element, that is to say, in the case of thisjoint, one workpiece is the electronic component, in particular thepower component, and the other workpiece is the supporting element forthe electronic component. The supporting element preferably comprises aheat sink for the electronic component.

The invention also provides a method for producing a heat-conductingadhesive joint between two workpieces, which has a higher thermalconductivity than a joint with a layer of adhesive to which aheat-conducting powder is added, and which comprises the steps of:

producing a layer of heat-conducting material,

which has two flat surfaces facing away from each other,

which, on each flat surface, has openings defined by voids in the layer,

which is arranged between the two workpieces in such a way that one ofthe two flat surfaces makes flat contact with one of the two workpieces,and the other flat surface makes flat contact with the other workpiecein each case, and

whose heat-conducting material is coherent from one of the flat surfacesin the direction of the other flat surface,

introducing liquid curable adhesive into the openings in the layerarranged in this way between the two workpieces, such that the liquidadhesive introduced wets each workpiece and

curing the adhesive introduced in this way.

According to this method, first of all a highly thermally conductivelayer is produced which is in contact with both workpieces, and onlythen is the layer bonded adhesively to the workpieces.

The heat-conducting layer is preferably and advantageously produced bythe steps of:

applying a paste to a workpiece and/or the other workpiece, said pastebeing composed of a mixture of a powder of heat-conducting material thatcan be sintered at a specific sintering temperature and a liquid,

bringing the two workpieces together in such a way that the paste islocated between the two workpieces and makes contact with bothworkpieces,

drying the paste and

sintering the dried powder by heating to the sintering temperature.

The layer of sintered powder can advantageously be electricallyconductive or nonconductive, depending on the selected heat-conductingmaterial of the powder and, moreover, can have the same advantages asare described above in relation to the layer of sintered metal powder,that is to say it can be produced easily, under certain circumstances itcan be sintered onto one workpiece or onto both workpieces to be joinedand, on its own, can already form a heat-conducting joint with one orboth workpieces, which assists the joint produced by the adhesive, it isinherently formed in such a way that it has openings defined by voids onits flat surfaces and is interspersed in the manner of a sponge withvoids, the voids being connected to one another and, in relation to acurable liquid adhesive, being capable of being so small that they exerta capillary suction action on this adhesive, and so on.

A higher density and therefore higher thermal conductivity of thesintered layer of the heat-conducting powder can be obtained if verymuch finer and/or very much coarser powder made of heat-conductingmaterial is added to the powder. Coarse-grained powder can be composedof metal or other substances with a good thermal conductivity, forexample of SiC or diamond.

A high density and therefore good thermal conductivity of the sinteredlayer of heat-conducting material can also be achieved with the step ofexerting a specific mechanical pressure on the powder during thesintering operation or after the completion of this operation.

Use is preferably made of a sinterable powder selected from the group ofmetals, in particular of the noble and semi-noble metals.

It is particularly advantageous to make use of a sinterable powdercomprising silver. If use is made of a powder comprising silverparticles, and the sintering of this powder is carried out in anoxidizing atmosphere, a sintering temperature between 100° C. and 250°C. is advantageously adequate for sintering. The sintering in anoxidizing atmosphere can also be advantageous in the case of sinterablepowders which contain substances differing from silver.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained in more detail by way of example in thefollowing description, by using the drawings, in which:

FIG. 1 shows two separate workpieces in cross section, to which in eachcase a paste composed of a sinterable heat-conducting material and aliquid is applied,

FIG. 2 shows the workpieces from FIG. 1 in the same representation butin the state in which they are brought together in such a way that thepaste forms a single continuous layer between the workpieces and makingcontact with both workpieces,

FIG. 3 shows the workpieces from FIG. 2 in the same representation butfollowing the drying of the paste and the sintering of the powder ofheat-conducting material to form a sintered layer, which is arrangedbetween the workpieces and makes contact with both workpieces,

FIG. 4 shows the detail A enclosed by a circle in FIG. 3 in an enlargedrepresentation,

FIG. 5 shows the detail A from FIG. 4 following the filling of theopenings and voids in the sintered layer with curable liquid adhesive,and

FIG. 6 shows the detail A from FIG. 5 following the curing of theadhesive.

The figures are schematic and not to scale.

DETAILED DESCRIPTION OF THE INVENTION

The heat-conducting adhesive joint according to the invention betweentwo workpieces will be explained in more detail using the example of apreferred specific production method.

FIG. 1 shows, as the initial stage of this method, two workpieces 1 and2 which are separated from each other and have mutually opposite andmutually shape-adapted, for example flat, surface sections 11 and 21,respectively.

For example, let the workpiece 1 be an electronic component, for examplea power component, in particular a power semiconductor component, andlet the workpiece 2 be a supporting element for the electronic componentwhich, in particular, can be a heat sink for this element or at leastcomprise such a heat sink.

Applied to the surface section 21 of the workpiece 2 and/or the surfacesection 11 of the workpiece 1 is a paste 5, which is composed of amixture of a powder of heat-conducting material that can be sintered ata specific sintering temperature T and a liquid. In FIG. 1, the paste 5is shown as applied to each workpiece 1 and 2, but it is also sufficientto apply the paste 5 to only one workpiece, for example the workpiece 2.

Following the application of the paste 5, the two workpieces 1 and 2 arebrought together in such a way that the paste 5 is located between thetwo workpieces 1 and 2, and the paste 5 makes contact as far as possiblewith the entire area of the surface section 11 and 21 of each workpiece1 and 2 and forms a thin layer 3′ between these sections 11 and 21,after which the intermediate stage of the method illustrated in FIG. 2has been produced.

The layer of the paste 5 is then dried and sintered after heating to thesintering temperature T.

For the drying of the paste 5 and also the subsequent introduction ofcurable liquid adhesive into the sintered layer, it is advantageous ifthe two workpieces 1 and 2 that have been brought together are pressedagainst each other, so that the paste 5 flows out of at least oneworkpiece, for example the workpiece 1, in a small bead 51, whichsurrounds this workpiece.

The drying of the paste 5 is carried out for example by permitting theliquid contained in the paste 5 to evaporate, which can be performed byheating the paste 5, for example during heating to sintering temperatureT, and/or at a negative pressure, for example in vacuum. The bead 51advantageously contributes to the liquid being able to evaporate withoutresidue and without forming bubbles.

Following the sintering of the dried powder, the intermediate stage ofthe method illustrated in FIG. 3 has been produced.

This intermediate stage has the sintered layer 3 of dried powderarranged between the surface sections 11 and 21 of the workpieces 1 and2, said layer having two flat surfaces 31, 31 that face away from eachother and a bead 30 which surrounds at least one workpiece and has beenproduced from the bead 51.

One of the flat surfaces 31, 31 adjoins the surface section 11 of theworkpiece 1 in a flat manner, the other adjoins the surface section 21of the workpiece 2 in a flat manner.

In order to increase the density of the sintered layer 3, a specificmechanical pressure p can be exerted on the powder between theworkpieces 1 and 2 during the sintering operation.

The sintering temperature T is determined by the powder material.

The detail A of FIG. 3 represented in enlarged form in FIG. 4 shows thestructure of the sintered layer 3 by way of example and schematically.

In FIG. 4, the obliquely hatched part 34 of the layer 3 containssintered powder of heat-conducting material, which is coherent from oneflat surface 31 in the direction 35 of the other flat surface 31 of thelayer 3.

All the unhatched white areas 32 of the layer 3 represent voids in thelayer 3. Although all these white areas would in each case have to beprovided with the reference symbol 32, for clarity only a few of theseareas have been designated with this reference symbol 32.

The voids 32 are interspersed through the layer 3 in the manner of asponge and, for the major part, are connected to one another even if notin the section plane illustrated. Voids 32 which adjoin a flat surfaces31 in each case define an opening 33 in this surface 31.

The method described up to this point is similar to the method describedin DE 34 14 065 A1 for producing an adhesive-free joint, and also themethod described in EP 0 242 626 A2 for producing such a joint, and allthe workpieces and materials for these workpieces which are specifiedthere, the liquid of the paste and the powder of the paste, and also thesintering temperatures and pressures specified there, can also be usedin the method described here for producing the adhesive joint accordingto the invention. The entire disclosure of DE 34 14 065 A1 and theentire disclosure of EP 0 242 626 A2 form a constituent part of thepresent application.

In the method disclosed by these two documents, the sintered layer iscomposed of metal and is sintered onto both workpieces. In addition, usecan be made of a sinterable powder selected from the group of metals forproducing the layer 3 according to the invention. In particular, use canbe made of a sinterable powder selected from the group of noble andsemi-noble metals. Such a layer 3 can be sintered onto the workpiece 1and/or 2. Use can advantageously also be made of sinterable powderswhich do not sinter onto a workpiece 1 and/or 2. The layer 3 can also beproduced with a heat-conducting nonmetallic sinterable powder, forexample a powder comprising ceramic material, SiC, diamond and so on.

It is beneficial if the surface section 11 or 21 of a workpiece 1 and/or2 is smooth, in particular polished, since in this case the particles ofthe sintered layer 3 come into particularly close contact with therelevant surface section 11 or 21 and ensure good heat transfer betweenworkpiece 1 and/or and sintered layer 3.

The openings 33 on the flat surfaces 31 of the sintered layer 3 arrangedbetween the workpieces 1 and 2 are then filled with curable liquidadhesive, which wets the surface sections 11 and 21 of the workpieces 1and 2.

On account of the structure of the layer 3, interspersed in the mannerof a sponge with voids 32 that are connected to one another, the fillingof the openings 33 with liquid adhesive can be carried out by suckingthis adhesive into the layer 3 until as far as possible all the voids 32and openings 33 are filled with the adhesive.

Sucking the adhesive in can be carried out by means of capillary actionof the interconnected voids 32 on the liquid adhesive and/or assisted bypressure. It is beneficial if the adhesive is as thin a liquid aspossible.

Liquid epoxy resin, for example, in suitable as a curable liquidadhesive 4.

The bead 30 is advantageous for the action of sucking the adhesive intothe layer 3, since it provides a relatively large area to the adhesiveto be sucked in.

Following such filling of the openings 33 in the layer 3, anintermediate method stage has been produced which is represented in FIG.5 which, like FIG. 4, shows the detail A from FIG. 3 enlarged. Theadhesive sucked into the layer 3 and filling the voids 32 and openings33 in the layer 3 is indicated by dashed horizontal lines in FIG. 5 anddesignated by 4. In the openings 33, the adhesive 4 wets the surfacesections 11 and 21 of the workpieces 1 and 2.

Following the curing of the adhesive 4 sucked in, the finished adhesivejoint according to the invention has been produced, and is representedin FIG. 6 which, like FIGS. 4 and 5, shows the detail A from FIG. 3enlarged. The cured adhesive filling the voids 32 and openings 33 in thelayer 3 is indicated in FIG. 6 as hatched obliquely and alternately thinand thick and is designated by 4′. In the openings 33, the curedadhesive 4′ adheres to the surface sections 11 and 21 of the workpieces1 and 2.

A particularly preferred embodiment of the adhesive joint according tothe invention comprises a layer 3 of sintered silver powder, which isparticularly suitable for the above-described method for producing thisadhesive joint since, according to the thesis mentioned above, silver isable to form sintered bridges even at low temperatures between 100° C.and 250° C., preferably between about 150° C. and 250° C.

In order to produce this adhesive joint, for example suitablefine-grained silver powder is stirred with a liquid, for example anorganic liquid, such as terpineol or ethylene glycol ether, to form apaste 5 which can be processed like a conductive adhesive paste.

Following application of the paste 5, for example by a dispenser, to atleast one of the two workpieces 1 or 2, which is, for example, asupporting element for an electronic component in the form of a chip,the other workpiece 2 or 1, the chip in the example, is placed on thepaste 5 in such a way that it flows out all around in a small bead 51.Then, during slow heating of the paste 5, the liquid can evaporatewithout residue and without forming bubbles, and the paste 5 can dry.

Following the drying, a layer 3 and a bead 30 of dried silver powderhave been produced between the workpieces 1 and 2 and are sintered.

For sintering silver at less than 250° C., an oxidizing atmosphere isimperative. Surprisingly, in the thin layer 3 of silver powder of lessthan 100 μm between the workpieces 1 and 2, the oxygen is able topenetrate in sufficiently quickly so that, even in areas of up to 5×5cm² or more, sintering of the silver powder takes place. For example, inareas of 2×2 cm², sintering of the silver powder takes place withinabout 15 minutes.

The finding has been made that, in an O₂-containing atmosphere, forexample in air, silver powder surprisingly begins to sinter even at lowtemperatures from 150° C. In this case, the sintering process manifestsitself in that the silver powder solidifies to form a foam with voidsand achieves a striking adhesive capacity. For example, a hot tip of apair of forceps adheres to the solidified sponge of silver powder underslight pressure. This adhesion also occurs on many smooth surfaces, suchas silicon, glass, corundum, polyimide, and is sufficiently strong forexample to sinter a chip onto glass and to cool it down to roomtemperature, to suck in curable liquid adhesive and, for example, forthe purpose of wire bonding. It is also true of silver that polishedsurfaces are particularly suitable for this purpose, since the silverparticles come into close contact with the surface. At high temperature,the adhesive capacity falls off again.

The sintered layer 3 of silver powder produced in this way isinterspersed in the manner of a sponge with voids 32 and has openings onits flat surfaces 31. Depending on the initial powder, the density ofthis layer 3 is between 40-50% by volume of silver and can be increasedfurther by adding very much finer and also very much coarser powder.Instead of silver, other substances with good thermal conductivity but alow thermal coefficient of expansion can be used as the coarse-grainpowder, such as SiC or diamond, for example in order to better match thecoefficient of thermal expansion of the sintered layer 3 of silverpowder to a chip.

A high silver density and therefore good thermal conductivity can alsobe achieved by applying pressure at 150° C. to 250° C., it beingpossible for pressure and time to remain far lower than in the knownmethod according to EP 0 242 626 A2.

The sintered layer 3 of silver powder exhibits a high capillary suctionforce, so that any thin liquid adhesive 4 can be sucked into the layer 3or can be pressed with pressure support.

The strength of the adhesive joint following the curing of the adhesive4 sucked in is then composed of the inherent strength of the sinteredlayer 3 of silver powder and that of the cured adhesive 4′, that is tosay can also become significantly greater than that of a pure adhesivebond. At high temperatures, the strength of the sintered layer 3 ofsilver powder dominates. Since, following the curing of the adhesive 4,the further ingress of oxygen into the sintered layer 3 of silver powderis prevented, the sintered structure of this layer 3 advantageously thenno longer changes at high temperatures.

The adhesive joint according to the invention can also be produceddifferently. For example, a layer 3 of heat-conducting material, forexample metal, can be used which has voids 32 in the form of holes, eachof which passes through the layer 3 from one flat surface 31 of thelayer 3 to the other and which is applied with one of the two flatsurfaces 31, 31 of this layer 3 flatly to the surface section 11 or 21of one of the two workpieces 1 and 2.

Each hole 32 defines an opening 33 in each case in each of the two flatsurfaces 31, 31.

This layer 3 can be, for example, a thin film of heat-conductingmaterial, for example metal, applied flat to the surface section 11 or21 of one of the two workpiece 1 and 2 and having the holes 32 andopenings 33 from the beginning.

Instead of the thin film, use can also be made of a layer 3 ofheat-conducting material which is applied to the surface section 11 or21 of one of the two workpiece 1 or 2 by vapor deposition, sputtering,thermal spraying and so on of the surface section 11 or 21, and in whichthe holes 32 and openings 33 are then produced, for examplephotolithographically and by etching the layer 3 with an etchant.

The holes 32 and openings 33 in the layer 3 applied are filled withcurable liquid adhesive 4 through the flat surface 31 of the layer 3that faces away from the surface section 11 or 21 of the one workpiece 1or 2, said adhesive wetting the surface section 11 or 21 of the oneworkpiece 1 or 2.

The surface section 21 or 11 of the other workpiece 2 or 1 is placedflat on the flat surface 31, of the layer 3 that is filled with theliquid adhesive 4, that faces away from this surface section 11 or 21 ofthe one workpiece 1 or 2, so that the liquid adhesive 4 wets thissurface section 21 or 11.

The adhesive 4 is then cured, so that it adheres to the two surfacesections 11 and 21 of the two workpieces 1 and 2.

What is claimed is:
 1. A heat-conducting adhesive joint that holds twoworkpieces together, the joint comprising: a layer of heat-conductingsintered metal powder which has two flat surfaces facing away from eachother and openings defined by voids throughout the layer, and which isarranged between the two workpieces in such a way that one of the twoflat surfaces makes contact with one of the two workpieces and that theother flat surface makes flat contact with the other workpiece whereinthe heat-conducting sintered metal powder is coherent from one of theflat surfaces to the other flat surface, and an adhesive, which fillsthe openings in the layer and which is adhered to both workpieces tohold the two workpieces together.
 2. The joint as claimed in claim 1,wherein the voids are interspersed throughout the layer in the manner ofa sponge.
 3. The joint as claimed in claim 1, wherein at least thosevoids in the layer which define openings on one flat surface of thelayer are joined to one another.
 4. The joint as claimed in claim 1,wherein the heat-conducting sintered metal powder is selected from thegroup of noble and semi-noble metals.
 5. The joint as claimed in claim4, wherein the heat-conducting sintered metal powder comprises silver.6. The joint as claimed in claim 1, wherein one of the workpieces iselectronic component and the other of the workpieces is a supportingelement for the electronic component.
 7. The joint as claimed in claim6, wherein the electronic component is a power component.
 8. The jointas claimed in claim 6, wherein the supporting element comprises a heatsink for the electronic component.
 9. A heat-conducting adhesive jointthat holds two workpieces together, the joint comprising: a layer of asintered heat-conducting metal powder that extends coherently from onesurface of the joint to an opposite surface thereof, said layer havingplural interconnected voids throughout, including in the surfaces of thejoint, that are connected to each other; and an adhesive that fills saidplural voids, including said voids in the surfaces of the joint, so thatsaid adhesive extends continuously from the one surface of the joint tothe opposite surface thereof, said adhesive being adhered to both of thetwo workpieces to hold the two workpieces together.
 10. The joint ofclaim 9, wherein said layer of sintered heat-conducting powder ishomogeneous.
 11. The joint of claim 9, wherein the adhesive is a curableliquid.
 12. The joint as claimed in claim 1, wherein the adhesive is acurable liquid.
 13. The joint as claimed in claim 1, wherein the voidsare interconnected to each other.